The present invention is concerned with a roller mill which can be used to mill material traditionally milled using ball mills. Ball mills are used widely to manufacture of powders in the cement industry and in the aluminium industry where they are used to mill bauxite, which is a particularly hard material.
Ball mills consume large amounts of energy. Typically, where a ball mill would use 80 kw/tonne to mill a material to a particular particle size, the roller mill of the present invention would use significantly less energy, perhaps as little as 4 to 5 kw/tonne.
There are certain waste materials which if milled to a sufficiently small particle size may be used in the manufacture of cementitious compositions. For example, lime stone may be incorporated in to cementitious compositions if it is reduced to a sufficiently small particle size. However, the energy consumption of a ball mill may render such reduction in particle size uneconomic. Similarly, there are products which could be used as part of a mixture to be burnt to generate power, if those products could be reduced to a sufficiently small particle size. It would therefore be advantageous to provide a mill capable of producing materials of small particle size for a modest consumption of energy.
In many instances, if these materials cannot be reduced in size economically they will be dumped in land fill sites, which incurs a cost and uses space required for materials which have no other useful purpose.
Another problem associated with ball mills particularly when miffing very hard materials, such as bauxite, is that the metal material of the balls wears leaving metal mixed with the milled product. This metal must then be extracted by passing the milled material between magnets, further increasing the cost of production.
A roller mill is known from GB 331877. In this mill grinding rolls are arranged to rotate about a vertical axis and to engage with a ring. The rollers are mounted on swinging arms, which are provided with biasing means to force the rollers against the ring before the centrifugal action of the mill comes into full effect. The product to be milled is delivered to the rollers by a feeder apparatus which rotates with the mill and is arranged to deposit product ahead of the rollers.
A multi-stage roller mill is known from Bulgarian patent no 37402 which includes three roller mills arranged in series and is provided with classifying means to remove particles of a size within a defined range between each milling stage.
A multi-stage mill is also known from Japanese Patent number 5096197. In this multi-stage roller mill a plurality of sets of rollers are mounted in a housing on a drive shaft common to each set. The roller mill described mills particles of solid substances which are entrained in a slurry. In addition to milling the solid particles the action of the roller mill ensures that solid particles are well dispersed in the slurry.
Roller mills of the prior art are not designed to mill hard substances, such as bauxite. However, in comparison to ball mills, roller mills use considerably less energy.
It would therefore be desirable to provide an improved roller mill.